In many refrigerant systems, capacity modulation is a desirable feature. Capacity modulation is especially desirable in air conditioning and heat pump applications when the load varies during the season. During periods of part-load operation, compressor capacity is reduced, resulting in a close match between the system capacity and the load which results in the system operating at a higher efficiency.
It is known to provide two or more compressors to be connected in parallel to a single refrigeration system and to selectively cycle either of the compressors off or on to provide variable capacity for the system. Operation of compressors in parallel poses certain problems arising from the different temperature and pressure levels which may be attained by each compressor. Generally, the pressure drop in the suction manifold leading to the compressor must be of a small value and uniform to each compressor. Oil equalization means must be provided between compressors since oil will tend to accumulate in the sump of the running compressor. Another problem confronting parallel compressor systems is that when a compressor in the system is deenergized condensed liquid refrigerant and oil may accumulate in the discharge system of the non-operating compressor. When the non-operating compressor is energized, the liquid refrigerant, if present in the discharge valve cavities, muffler, and discharge line, puts an abnormal strain on the starting compressor that can, because of slugging, result in discharge valve damage and premature compressor failure.
In some prior art teachings, such as that disclosed in U.S. Pat. No. 3,126,713-Parker, a multiple compressor system includes a check valve for preventing accumulation of liquid refrigerant in the discharge system of one of the compressors. When only one compressor is protected from refrigerant condensing in its discharge system, the sequence of operations must always include energizing the primary compressor in response to a first condition and both primary and secondary compressors in response to a second condition. If the secondary compressor were to operate independently of the primary compressor, refrigerant condensation would result in the then-idle primary compressor discharge system. For this type system to be effective, the primary compressor must be operating when the secondary compressor is energized.
By the present invention, a multiple compressor refrigerant system includes valve means associated with each compressor which assures that condensed refrigerant and oil will not collect in the discharge system of any compressor in the system when it is idle.